Evolution of the EEG in children with Rasmussen's syndrome.

PURPOSE: The early diagnosis of Rasmussen's syndrome (RS) is often difficult, with differentiation between RS and focal cortical dysplasia (FCD) at epilepsy onset problematic. This study reviewed electroencephalography (EEG) in the two conditions for early indicators of either pathology. METHODS: All children with either suspected RS or with unilateral FCD undergoing evaluation for epilepsy surgery between 1992 and 2005 were identified. Case notes and standard EEG recordings were reviewed. EEG findings were compared where available at <3, 3-6 months, and 3-5 years after seizure onset. KEY FINDINGS: Nineteen children with RS and 17 with FCD were ascertained. In EEG studies performed <3 months after seizure onset, 50% (5/10) of the RS group showed background abnormalities, with 80% of these (4/5) showing persistent high-amplitude delta activity over the affected hemisphere. This compared to 66% (6/9) of the FCD group with 17% (1/6) showing marked background asymmetry. By 3-6 months after seizure onset, independent interictal abnormalities over the nonaffected hemisphere were seen in 25% (2/8) of the RS group and by 3-5 years in 62% (5/8) compared to none in the FCD group at any time points measured. These independent contralateral interictal abnormalities were notably associated with a significant decline in cognitive skills over time. SIGNIFICANCE: No specific EEG changes at diagnosis of epilepsy were identified to help differentiate between RS and FCD. Emerging persistent delta activity over the affected hemisphere with contralateral normal background rhythms, followed in due course by independent interictal epileptiform abnormalities over the unaffected hemisphere may support the diagnosis of RS as the condition evolves, and highlight the risk of overall cognitive dysfunction.